We propose to continue our research upon the processes of viral infection and replication, with focus upon the infection of Escherichia coli by the small bacteriophage phi X174. We seek to understand these processes - viral adsorption, DNA penetration and replication, viral transcription, progeny DNA synthesis and viral assembly, and cell death and lysis - in molecular terms. Because the amount of genetic information and therefore the number of new molecular forms introduced into the cell by such a small virus is very limited (less than 10 new molecular species) we can attempt to establish the characteristics of the viral-coded molecules and to trace in detail their interactions with each other, with the viral genome, and with various host elements. BIBLIOGRAPHIC REFERENCES: Espejo, R.T. and R.L. Sinsheimer. The process of infection with bacteriophage phi X174. XXXIX. The structure of a DNA with restricted binding of intercalating dyes observed during synthesis of phi X single-stranded DNA. J. Mol. Biol. 102:723-741, 1976. Fukada, A. and R.L. Sinsheimer. The process of infection with bacteriophage phi X174. XXXVIII. Replication of phi X174 replicative form in vivo. J. Virol. 17:776-787, 1976.